1. Field of the Invention
The present invention relates to liquid crystal displays, and particularly to a liquid crystal display that adopts a hold-type display scheme.
2. Description of the Background Art
In liquid crystal displays, tailing and after image may occur in the displayed moving images. When such troubles are due to the response time of the liquid crystals, there are known methods that improve the response time of the liquid crystal panel, or that use the overdrive function. In the overdrive function, when the gray level of the moving picture displayed on the liquid crystal panel does not achieve the target gray level, the voltage applied to the liquid crystal panel is temporarily raised to compensate for the lack of the applied voltage, so as to achieve the target gray level.
It is thus possible to achieve the target brightness within one frame by applying these methods, but the so-called hold-type display scheme has a peculiar problem that the boundaries of objects appear blurred when displaying moving images, because it provides no dark display between frames like the impulse-type display scheme.
Pseudo-impulse driving schemes have been developed in order to solve the problem, including a scheme called black frame insertion (black writing) where black images are inserted between frames, as illustrated for example in FIG. 50 of Japanese Patent Application Laid-Open No. 2006-178488 (which is hereinafter referred to as Patent Document 1), and a scheme where backlight brightness is controlled to insert black display periods.
However, while adopting the black frame insertion (black writing) scheme improves the display of moving images, inserting black of the minimum brightness between frames reduces the brightness in the entire screen.
There is a method that increases backlight brightness to solve the reduction of brightness, but it leads to problems such as increased power consumption and increased black brightness.
For techniques that adopt the black frame insertion (black writing) while suppressing the reduction of brightness, Patent Document 1 and Japanese Patent Application Laid-Open No. 2001-296841 (which is hereinafter referred to as Patent Document 2), for example, disclose methods in which black is written when the input data is below a defined value, and white or a color of a gray level close to white is written when the input data exceeds the defined value.
Also, Japanese Patent Application Laid-Open No. 2000-029442 (hereinafter referred to as Patent Document 3) discloses a method in which an extreme value (white or black) and a halftone are displayed in alternate frames to display the same gray level as the input data.
In this way, for the problem of image blur in displaying moving images, which is peculiar to the hold-type display scheme, various attempts have been made to solve the problem with no reduction in brightness, no increase in power consumption, and with no increase in black brightness, but such attempts involved problems leading to increased costs, such as increased circuit scale, need for increased frame memory capacity, and need for higher-speed communication with the frame memory.